The Earth’s Soils Could Contribute More To Climate Change Than Previously Thought

The Earth’s soils play an important part in managing climate change by storing carbon, and thus keeping it out of the atmosphere. But research published Wednesday in Nature suggests that as global temperatures rise, the ability of soils to perform that service goes down.

The researchers tested 22 different soil samples from different points along the climatic gradient, from the Arctic all the way to the Amazon. It’s the microbes in each sample that determine how much carbon the soil stores versus how much it releases over a given time. So the researchers applied different temperature changes to each sample over a 90-day period to see how the mircobes would respond.

The question is an important one because soils and their microbes around the world store more than twice as much carbon as the atmosphere does, and release around 60 billion metric tons of that carbon into the atmosphere every year. Some of that carbon is absorbed by other parts of the planetary ecosystem — forests or the ocean, for instance — and eventually makes its way back to the microbes again before being re-released into the atmosphere.

But if temperature changes alter the microbes’ respiratory behavior, they could be releasing enormous amounts of carbon at greater rates, meaning there would be more carbon in the atmosphere at a given time. That, in turn, would exacerbate climate change.

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Up until now, the general assumption has been that the microbes would probably acclimate to temperature changes: after briefly changing the way they take in and release carbon, the microbes would get used to the new temperature and settle back into their previous pattern.

But Kristiina Karhu from the University of Helsinki, the Nature study’s lead author, told the BBC that based on the researchers findings, that wasn’t what happened.

“We show that for these 22 soils, this type of acclimation of microbial respiration doesn’t really happen,” she said. “Sometimes the opposite happens, in response to long-term temperature change, the microbes enhance the short term effect of temperatures so that the sensitivity of respiration gets actually higher.”

In other words, as the temperatures went up, many of the microbes tended to release more carbon.

The effect was particularly pronounced in soils from northern climates, such as the Arctic and boreal regions. “Microbial community response increased the temperature sensitivity of respiration in high latitude soils by a factor of 1.4 compared to the instantaneous temperature response,” according to the study. As Karhu pointed out, that’s worrying because northern soils and their microbes store more carbon than soils at other latitudes.

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“The soils that had this enhancing response were also soils that had a high carbon to nitrogen ratio,” Karhu said. “So it could be something in this interaction between carbon and nitrogen cycles, and there are some studies that suggest that maybe the enzymes related to nitrogen may be more temperature sensitive than the carbon related enzymes.”

It’s an example of what climate scientists call a feedback loop. Human beings pump more carbon into the atmosphere, which drives up global warming. But then that warming also changes the Earth’s natural ecosystems, so that the natural carbon cycle also begins dumping more carbon into the atmosphere than it did before, driving global temperatures up still further.

As hard as scientists work to build models to accurately project ecosystem changes and the effects of global warming, it’s a horrendously complex system — and this research suggests the models are underestimating the amount of carbon northern soils especially will release as global warming proceeds.

“Big advances have been made in recent years, and there are now models that simulate key microbial processes,” said Iain Hartley, another author of the study, told the BBC.”We have a great opportunity to really advance this subject, and improve predictions of rates of carbon dioxide release from soils under global warming, but there is still a huge amount that we need to understand better.”

The complexities abound: some forms of fungi grow more profusely in hotter temperatures, and one type of fungi in particular — ecto- and ericoid mycorrhizal (EEM) fungi, to be specific — can affect the ability of soil to store carbon. Because EEM fungi changes the decomposition process in soil, places heavy in EEM fungi end up storing carbon longer in the ground. And the difference can be dramatic, altering the amount of carbon released into the atmosphere at a given time by as much as 70 percent.